Computer graphic input devices which employ a totally internally reflecting prism have been described by Robert E. Mueller in U.S. Pat. No. 3,846,826 and Richard Greene in U.S. Pat. No. 4,561,017. In both of these devices, the input sensitive drawing surface of the prism is imaged from a center of projection, or viewpoint, which is at a finite distance from that surface. In Mueller's device, the viewpoint is defined by the secondary nodal point of the flying spot scanner lens, and in Greene's device it is the primary nodal point of the camera lens. In both cases, the angle of view of the drawing surface must everywhere exceed the critical angle, in order for total internal reflection to occur. As long as that angle of view is greater than the critical angle at the point on the drawing surface which is closest to the viewpoint, then it must be greater at all other points as well.
However, since the angle of view is closer to the critical angle at the nearer portions of the drawing surface than at the more distant portions, the sensitivity of the device is greater at those nearer portions. That is because less disturbance of the drawing surface is sufficient there to prevent total internal reflection. Also, drawing tools are viewed there less obliquely than at the more distant portions and so provide a stronger signal. Thus, users of such devices find that the parts of the drawing surface nearer the projection center are overly responsive to water droplets and other surface contaminants to which the more distant parts are relatively immune. By the same token, lines drawn from the nearer to the farther parts of the drawing surface appear to decrease in width, even when the actual drawing tool has a constant width and is applied with a constant pressure. The larger the drawing surface is relative to its distance from the projection center, the more severe is this nonuniformity.
A method of equalizing the sensitivity across the drawing surface employed in modifications of Greene's device is to curve the surface of the prism through which the drawing surface is viewed. The appropriate curvature causes the rays converging towards the projection center to be parallel within the prism, so that their angle with the drawing surface is everywhere equal. However, prisms employing such a curved surface are considerably more difficult to manufacture, and hence expensive, than flat surfaced prisms.
In the embodiment of the device described in the prior Greene patent, the camera forms an image of the drawing surface on a plane which is not parallel to that surface. That fact, along with the finite distance of the viewpoint, causes the image to suffer from keystone distortion. Thus a rectangle on the drawing surface is imaged as a trapezoid. As described in the Greene patent, column 8, lines 1 to 65, to compensate for that distortion, the camera electronics can be modified to yield a trapezoidal raster scan.
The curved prism discussed above and used for sensitivity equalization does nothing to prevent keystone distortion, since the rays which come from the more distant portions of the drawing surface traverse a greater distance while converging than those which come from the nearer portions. Thus a camera modified for electronic keystone correction must still be employed with such a curved prism.
In Mueller's device, no keystone distortion occurs since the plane of the flying spot scanner's raster is parallel to the drawing surface. Greene's patent also mentions the possibility of orienting the camera such that its light sensitive surface is parallel to the drawing surface (see column 9, lines 2 to 8). However, such an implementation would require a special purpose lens.
Another optical method for preventing keystone distortion is described in both Richard Edmund Reason's U.S. Pat. No. 2,354,614 and John F. Bartucci and James A. Horton's U.S. Pat. No. 3,655,260. In both of their inventions, a collimating lens is interposed between the tilted object plane and the imaging lens. The collimator is placed such that its focal point is coincident with the primary nodal point of the imaging lens. Thus, the principal image forming rays are all parallel in the space between the object and the collimator, and the image is a parallel projection free of keystone distortion.
The obvious disadvantage of this method is that the collimator must be wider than the object to be imaged and higher than the height of that object times the cosine of its tilt angle. Since keystone distortion is only a significant problem in Greene's device when the drawing surface is large compared to its distance from the camera, this method would require a very large collimator in such an application. Thus it would not be cost effective for keystone correction alone and the motivation to use such an approach would be absent.
While the imaging optics described in Bartucci and Horton's patent are simpler than Reason's, they also require that the tilted object plane be located one collimator focal length away from the collimator. Thus it would seem that a graphic input device employing their method would have to encompass an optical path length between the drawing surface and the camera twice as long as the collimator's focal length. In order to maintain the original size of the device, this would require a short focal length collimator, which is even costlier. Furthermore, Bartucci and Horton's method would seem to require a tilted camera lens.